A system in which letters are input by handwriting gestures in the air has been considered (see Non Patent Literature 1 below). In this system, when a user wearing a wearable camera on his/her head makes a finger gesture to write a letter in the air in front of his/her eyes, a motion of his/her hand is analyzed, and then the letter is identified from the analysis result. Also in this system, at the timing to begin writing a letter and at the time to finish writing the letter, the user has to perform an explicit operation to the system to clarify which letter to be recognized.